2.4. Procedure

Subjects were seated on a comfortable chair. The coil was placed tangentially to the scalp over the left (dominant) hemisphere with the handle pointing backwards and laterally at 45° from midline, approximately perpendicular to the central sulcus, inducing a PA current in the brain. The motor hot spot, defined as the larger MEP in the contralateral muscle with the lowest intensity, was determined by moving the coil in ∼0.5 cm steps around the hand motor area. The motor hot spot was checked for AP coil orientation as well. There was no difference in the hot spot between PA and AP coil orientation, as has been reported previously (Arai et al., 2005, Jung et al., 2012, Sakai et al., 1997). The position was marked with a red pen on an elastic cap placed on the subject’s head. The coil was always maintained in the same position during each condition of the experiment.

The resting motor threshold (RMT) and active motor threshold (AMT), defined as the lowest stimulation intensity that evoked MEPs >50 μV and >200 μV respectively in at least 5 out of 10 consecutive trials, was determined at the hot spot for each pulse width (30, 60, 120 μs) and coil orientation (PA and AP). For AMT measurement, subjects were asked to maintain approximately 10% contraction of the muscle. Three pulse widths were used for investigating motor cortical responses compared to four for peripheral motor axons (see below), because three pulses has been shown to be sufficient for estimating cortical SD time constants (Peterchev et al., 2013) and because the experimental sessions we already long with many conditions, thus we were keen to avoid the potential for discomfort and fatigue of participants.

IO curves were obtained for each pulse width and orientation at rest and in active contraction. MEPs were recorded starting from the minimum to the maximum intensity, in steps of 2% output. The minimum and maximum number of distinct pulse amplitudes per subject and condition were 14 and 45 respectively, depending of the pulse width. The maximum intensity was determined either by the maximum the subject can tolerate or by the maximum that can be reached by the device. The relationship between intensity and the MEP size was fitted by a cumulative Gaussian Sigmoid function after log-transformation (Peterchev et al., 2013).

At the end of the active contraction condition, 15 additional MEPs were recorded during contraction at 110% AMT for each pulse width and orientation in order to measure the onset latency of MEPs.